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GRC Transactions, Vol. 35, 2011

Investigation of Geothermal Resource Potential in the Northern , and New Mexico

Elisabeth Easley, Laura Garchar, Mitchell Bennett, Banks Beasley, Rachel Woolf, and Joyce Hoopes Colorado School of Mines, Golden, Colorado

Keywords Introduction Rio Grande Rift, Poncha Springs, Embudo Fault Zone, Taos Plateau, noble gas isotopes, geochemistry, geochemical mod- The Rio Grande Rift stretches over 1,000 km from Chihuahua, eling, geophysics, geothermal exploration Mexico to northern Colorado and exists in a setting that experi- enced nearly continuous deformation in the Cenozoic (Keller et al., 1990). Deformation during the Late Cretaceous Laramide ABSTRACT Orogeny was characterized by NE-SW compression, and the formation of foreland basins and uplifted fault blocks (Corbitt and Geothermal anomalies within the Rio Grande Rift are associ- Woodward, 1973; Drewes, 1978; Seager and Mack, 1986). Around ated with transfer and scissor faults between successive basins, 32 to 30 Ma, a shift to backarc extension onset rift inception, and intrabasin rift faults, where range-front faults are offset in accom- was followed by alkali rhyolite and basaltic andesite magmatism modation zones, volcanic complexes, and possibly where blind (29 to 30 Ma) that produced large shield volcanoes, fissures, intrusions of magma have risen to mid-crustal depths. This study thick flood-basalts, cinder cones, and tuffaceous ash layers scat- is part of the National Geothermal Student Competition sponsored tered about the San Luis Basin and the Rio Grande Rift (Aldrich by the National Renewable Energy Laboratory and aims to as- et al., 1986). The rift is composed of a series of north-trending sess the potential for geothermal resources in the San Luis Basin extensional basins that consist of asymmetric grabens (Olsen et of southern Colorado and northern New Mexico. A preliminary al., 1987) bounded on one side by north-striking normal range- analysis of existing data was used to identify two areas of interest front faults. for field investigations located near zones of structural complexity The San Luis Basin extends from approximately 10 km at Poncha Pass, Colorado and near Taos, New Mexico, represent- south of Salida, CO to over 80 km south of the Colorado-New ing the northern and southern ends of the basin. Mexico border near Taos (Kellogg, 1999). The basin comprises A suite of techniques including geological, geophysical, hy- two east-tilting half grabens separated by the north-trending drologic, and geochemical applications was used. Geochemical Alamosa Horst (Brister and Gries, 1994), which divides the modeling of existing data for the Taos Valley and Valles basin into the Monte Vista graben to the west and the Baca in New Mexico simulate the mixing of thermal and groundwater graben to the east. Near the Colorado-New Mexico state line end-members to represent geochemical processes potentially the basin is divided into the San Luis Valley to the north and occurring at depth. Noble gas isotope data suggest the presence Taos Plateau to the south. The San Juan Volcanic Field marks of mixing in a three end-member model system composed of at- the western boundary of the basin, while the east side is bounded mospheric, crustal, and components. isotope data by the Sangre de Cristo normal fault system, marking the front from Poncha Springs, Colorado yields a R/RA value indicative fault system of the Sangre de Cristo Range (Kellogg, 1999). of a mantle-sourced gas signature that may be associated with Strata in the San Luis Basin are tilted predominantly to the east, crustal penetrating faults, ongoing rift activity, and/or the Aspen but adjacent basins within the rift are separated by complexly anomaly. The use of noble gas isotopes for this research implies faulted zones, across which basin tilting is reversed (Bauer et that there is a direct and unique relationship between noble gas al., 2004), coinciding with zones of structural weakness asso- compositions and structural, magmatic, and tectonic geological ciated with Laramide and pre-Laramide tectonism. In several phenomena. Geophysical data indicate faults that serve as conduits areas, these complexly faulted zones localize magmatic activity and barriers to upwelling thermal water near Poncha Springs. (Russell and Snelson, 1994).

761 Easley, et al.

Background Results Geophysics Poncha Pass is located in south-central Colorado between the Upper Arkansas Basin and the San Luis Basin. It represents The Colorado School of Mines geophysics department col- a structurally complex transition between the two basins, and lected several sets of data along Hot Springs Rd (CO Road 115) lies in an intersection of two sets of crossing fault patterns (Coe near Poncha Springs, CO as part of summer field camp (Colorado et al., 1982; Grauch and Drenth, 2009). Poncha Hot Springs are School of Mines, 2010). The Sand Gulch area was investigated located well above the valley floor on the northern side of the pass as part of the NREL National Student Geothermal Competition along the intersection of an east-west to southwest trending fault in April of 2011. At the Poncha Pass field area, the Hot Springs and a north to northwest-trending fault. Quaternary, Tertiary, and Road direct current (DC) resistivity and self-potential surveys Precambrian rocks are exposed in the Poncha Pass area. identified two faults that coincide with evidence of upwelling Gravity measurements support the interpretation of east- water. The southernmost fault apparent in the resistivity profile trending faults at Poncha Hot Springs, and estimate about 1,000 appears to be a splay off of a main fault when compared with the ft of displacement along the fault just north of the hot springs. It aeromagnetic results. An additional smaller fault was inferred on has been hypothesized that the geothermal resource is found in the north end of the line and is considered to be a fault leg that Precambrian rocks and confined to faults and fractures (Coe et continues eastward. The fault identified at the southern end of the al., 1982). Aeromagnetics has identified faults that correspond Sand Gulch line appears to be the same fault, though the aero- to geologic mapping, and supports the idea of a faulted basement magnetic data suggests that this eastern fault expression is also a block buried underneath sediments between the town of Poncha splay. Although the two survey lines crossed the same fault, the Springs, to the north, and the hot springs (Grauch and Drenth, eastern survey did not reveal any evidence for upwelling water 2009). Barrett and Pearl (1976; 1978) sampled the Poncha along either of the faults found there. Springs area and interpreted reservoir temperatures ranging from 96-145 °C based on silica and alkali chemical geothermometers. Geochemistry Mixing is expected to occur between the deep geothermal reser- Geochemical data were collected at locations (Figure 1) near voir and local groundwater, but the degree of mixing is not well Taos (red points) and Poncha Springs (green points). The samples constrained and thus reservoir equilibration temperatures should were collected from thermal springs, cold springs, and surface be used with extreme caution. Four temperature gradient holes waters. Low precipitation for 2011 rendered sampling of springs have been drilled and showed anomalous temperature gradients difficult because many documented springs were not flowing in ranging from 56.4 to 65.6 °C/km. Coe et al. (1982) proposed the April. Geochemical data for the Rio Grande Gorge springs from heat source is a rift-related elevated geothermal gradient, however Bauer et al. (2007) and for the Valles Caldera springs from Goff helium isotope data from Karlstrom et al. (2011) and this research and Grigsby (1982) were used to model mixing of groundwater provide evidence of mantle-sourced gases in water samples col- and thermal end-members. The dataset of Bauer et al. (2007) has lected from Poncha hot springs. not been extensively analyzed in the literature. Existing datasets The geology of the southern San Luis basin is distinctly dif- (Bauer et al., 2007; Goff and Grigsby, 1982) were used to construct ferent from Poncha Springs, with massive Neogene basalt flows preliminary simulations of fluid mixing chemical reaction path incised by the Rio Grande, and the adjacent Valles Caldera on its models using The Geochemist’s Workbench (Bethke, 2008) that southwestern margin. This study focuses on the northeastern ex- define a reaction pathway for mixing of the Valles Caldera water tension of the Jemez lineament, the Embudo fault transfer zone end-member with an end-member representative of groundwater that separates the San Luis and Española basins of north-central in Taos County. The data collected by Bauer et al. (2007) are plot- New Mexico, which underwent significant Laramide shortening, ted on a Piper diagram, a basic ternary plot of primary anions and Miocene to Holocene extension, and episodic volcanism since cations, to emphasize the position of the end-members presented the early Oligocene (Bauer and Kelson, 2004). Three major on this diagram, as shown in Figure 2. fault zones are reported in this area and are of interest for geo- Table 1. Valles Mixing Model Parameters. thermal energy development because they may serve as conduits or barriers for hydrothermal fluid flow and storage. These fault Valles Mixing Model Parameters zones include the strike-slip Embudo transfer fault zone, the 8 Basis Reactants mole km-wide north trending strike-slip Picuris-Pecos fault system, H2O 1 free kg H2O 55.51 and the dip-slip Cañon section of the Sangre de Cristo Range Na+ 2100 ppm Na+ 0.001653 front fault. Seismic interpretations by Reiter and Chamberlin Ca++ 0.01 ppm Ca++ 0.00078 (2011) suggest that upwelling of the mantle- occurs K+ 777 ppm K+ 0.000537 near the Rio Grande Rift and the Great Plains craton boundary. Mg++ 0.01 ppm Mg++ 0.000494 Previous reports on the springs and hydrology of Taos County H+ 9 (pH) H+ 0.000237 are numerous (Bauer, 2007; Summers, 1976; White and Kues, HCO3- 323 ppm HCO3- 0.0042 1992; Garrabrant, 1993; Johnson, 1998; TetraTech, 2003; and Cl- 3618 ppm Cl- 0.00026 Benson, 2004). The report of Bauer et al. (2007) was used for SO4-- 66 ppm SO4-- 0.00027 this research, and includes geochemical data in the Rio Grande SiO2(aq) 820 ppm SiO2(aq) 0.00011 Gorge near Taos and the Embudo Fault Zone. Temp 126 C Temp 13 C Data from Bauer et al., 2007

762 Easley, et al.

reactions of the defined system became buffered by some mineral or gas phase. This may be due to the lack of dissolved CO2 gas partial pressure or fugacity data for the Valles caldera, as Goff and Janik (2002) report that the gas component of dry gas samples from this hydrothermal system ranges from 96.9 to 99.0 mol %. The data of Goff and Janik (2002) does not include the ratio of dry gas to water, or dissolved gas, and the geo- chemical modeling approach to represent this system allows for input parameters of dissolved gases only. Future work to mea- sure a suite of dissolved gases, including CO2 and H2S is underway. The Rio Grand Gorge-Valles Caldera reaction path model also reveals infor- mation regarding the scaling potential of thermal waters. Mineral saturation is shown in Figure 4, and is defined as (Q/K), where Q is the reaction quotient and K is the solubility product. A Q/K Figure 1. Geochemistry sample locations from National Student Geothermal Competition. value greater than one indicates that a

I Cisnerso J Sunshine Trail K Desagua Trail L S of Sheep M Gaging Station S O Bear Crossing spring zone P Feisenmeere Middle A North Big Arsenic 2 B Little Arsenic C Rael (Stark) D Black Rock Hot E Godol North 1 spring zone G Godol North 2 spring zone H Godol South spring zone 1 I Godol South Spring zone 2 J Manby south pool K Taos Junction South L Rio Grande (Klauer) M Acequia de los Ojos O Souse Hole cienega P Valles Caldera (Baca 4) A Valles Caldera B Soda Dam Springs C Main Jemez Springs D Travertine mound spring

Figure 2. Piper diagram of Rio Grande Gorge and Valles Caldera Waters. Figure 3. Piper diagram of reaction pathway between meteoric and mag- matic end-members.

The REACT module in GWB was used to construct a fluid mineral phase is oversaturated in solution and will precipitate. mixing reaction pathway of a chemical system defined in Table 1. Calcite (red curve) and quartz (dashed black line) mineral This reaction path model simulates an equilibrated hydrother- saturation is shown in Figure 4 and provides evidence for mal reservoir fluid mixing with a fluid representative of a local quartz saturation with increasing reaction progress. In Figure equilibrated groundwater. The results are presented on a Piper 4, quartz remains slightly oversaturated for the entire reaction diagram in Figure 3, and simulate a chemical pathway as thermal progress, which may be an artifact of the model or a result of reservoir equilibrated water mixes with cold local groundwater to the slow kinetics of quartz precipitation. The model predicted produce a water chemistry similar to water chemistry measured the mineral with the highest potential for scaling is quartz, and at the Godoi South Spring and Rio Grande by Bauer et al. (2007, precipitation as thermal fluids ascend may be controlled by pH. Figure 2). Both reaction paths plotted on the ternary diagrams The calcite mineral saturation curve drops quickly during the (Figure 3) indicate linear mixing trends and future work to cal- reaction progress and remains undersaturated for the majority culate the mixing fraction of hot and warm springs in the Taos of the reaction path. This is most likely due to insufficient dis- Valley is in progress. The reaction pathway after 300 simulated solved gas data for model parameters because calcite scaling

763 Easley, et al.

tribution to the total mass balance of noble gases in the is negligible (Ballentine and Burnard, 2002). The goal of using Quartz 1 noble gases for this research is to define sources of geothermal heat and understand the relationship between noble gas content and geological phenomena. Noble gas geochemistry studies were performed, and reported in Table 2 as the ratio of 3He/4He in the sample to the ratio of 3 4 Calcite He/ He in the atmosphere, or R/Ra values for helium, and may indicate different mantle signatures in springs distal and proxi- mal to major tectonic structures. The use of a three end-member system is the basis for the conceptual model defined in Figure 6, 4 4

Mineral Saturation (Q/K) which plots R/RA versus the ratio He/Ne*air, or the He/Ne in the sample normalized to the He/Ne in the atmosphere. Samples attributed to the atmospheric end-member plot close to an R/RA value of 1.0 shown by the blue horizontal trend (Figure 6), but may also vertically deviate from 1.0 if they are enriched by tritiogenic 0 0.1 0.2 3He. Conventionally speaking, samples derived from the mantle Reaction progress (mid-ocean ridge basalt, MORB) end-member have a high R/ Figure 4. Mineral saturation along the reaction path. RA value around 8.0, whereas crustal end-member varieties are indicated by a R/RA value of 0.02. is probable for this system, as indicated by travertine mounds These three end-members illustrated in Figure 6 are respon- near the caldera. sible for the production of noble gas isotopes, however only the Noble gas isotopes in the crust are useful tracers in hydrother- magmatic and crustal end-members may be responsible for the mal systems, due to their relative abundances, chemical inertness, and unique isotopic characters. They can therefore be used to Table 2. Noble Gas Isotope Results. determine the source of fluids, the environment of fluid origin, manner of physical transport, and phase changes associated with Noble Gas Isotope Results 4He Ne chemical interactions in the crustal fluid system (Ballentine and Sample Location [cc STP/g] R/RA RC/RA [cc STP/g] Burnard, 2002; Ballentine et al., 2002). Figure 5 (not to scale) Manby Spring 2.024E-6 0.32 0.31 1.257E-7 is a conceptual model that illustrates three sources of noble gas isotopes attributed to atmospheric, mantle, and crustal radiogenic/ Big Arsenic Spring 4.885E-8 0.99 3.46 1.686E-7 nucleogenic processes, which are defined as end-member compo- Joyful Journey 3.733E-7 0.73 0.70 1.057E-7 nents for the purpose of this research. The release of 3He/4He in Poncha Spring 1.131E-7 1.90 2.16 8.758E-8 Waunita 3.725E-7 0.17 0.10 1.074E-7 the mantle has a constant and anomalous value of approximately 1 -5 Cottonwood 9.300E-6 0.58 0.58 na 1.2 x 10 compared to that of tectonically “old” and stable crust, 1 which may have a 3He/4He production ratio on the order of 10-8 Mt Princeton 3.540E-7 0.57 0.64 na 1 (Ballentine and Burnard, 2002). Additional sources of noble Poncha 1.040E-6 2.16 2.24 na 1 gases include interplanetary dust particles (IDP), cosmogenic Waunita 6.390E-6 0.18 0.18 na 1 reactions, and anthropogenic production, however their con- Data from Karlstrom et al., 2011 (in review)

Atmospheric Component

3, 4 He R/RA = 1.0 20, 21, 22 Ne 36, 38, 40 Ar QATM MORB

ASW Salinity, T, P

R/RA = 0.98

Tritiogenic 3He

Radiogenic/Nucleogenic Component Atmospheric 235, 238 U Æα4He* 232ThÆα4He* Crustal Magmatic Component 40 K Æec40Ar* (Also β‐ 40Ca) 24Mg Æ (n,α) 21Ne* He‐dominated 3 He 6Li Æ (n, α) 3H (β‐) 3He* R/RA = 8 (MORB) QCRUSTAL R/RA = 0.02 QMAGMATIC

4 Figure 5. R/Ra Conceptual Model of Noble Gas Isotope Reservoirs. Figure 6. R/RA vs. He/Ne * Air.

764 Easley, et al. production of heat associated with a geothermal anomaly. Further, the MORB end-member for this conceptual model is hypotheti- Helium Budget cal and would not be reached in this system because an upper 0 100 crustal pluton does not have the same 3He/4He ratio as a MORB. CSM data, 2011 Radiogenic and nucleogenic reactions in the crust account for Karlstrom et al., 2011 approximately 50 to 75% of the total crustal heat budget (Bal- 20 lentine and Burnard, 2002; O’Nions and Oxburgh, 1983; Turcotte 80 and Schubert, 1982), and the remainder may be attributed to % A localized phenomena, for example, plutons at depth, volcanism, ir

40 S extensional basins, etc. It is important to again note in Figure 6 le 60 a t t n u that neither the crustal or magmatic end-member is reached, thus a r a M all fluid samples have some degree of noble gas mixing with the te d % magmatic and atmospheric end-member. It is possible that noble W 60 40 a gas isotopes may be slowly leaking from the mantle along crustal te penetrating structures, and traveling to the surface by advection r without further isotopic fractionation (Ballentine and Burnard, 2002; Bickle and McKenzie, 1987). Understanding the different 80 20 sources of geothermal heat and identifying mantle signatures, which often are masked by crustal processes, are important aspects of geothermal exploration because it is essential to discern high 100 0 radiogenic crustal heat flow from heat flow of magmatic origin from intrusions into the crust or lithospheric thinning from exten- 0 20 40 60 80 100 sion (Ballentine et al., 2002). % Crustal Mixing trends are also observed in Figure 7, which plots RC/ Figure 8. Ternary model of Helium Budget. RA, or the R/RA corrected for neon, versus excess helium in cc STP/g. Excess helium was calculated by subtracting the atmo- spheric helium contribution, separating the crustal and mantle components. Figure 7 suggests mixing between samples that 1 displays a map of sample locations in the San Luis basin and plot in a linear trend between the crustal and mantle (MORB) a ternary plot of each sample locations to show a correlation be- end-members. Data represented by purple points was collected tween tectonic and rift-related structures. It is important to note as part of the National Student Geothermal Competition and that from this preliminary suite of data, that there appears to be orange points represent data collected by Karlstrom et al. (2011). a direct and unique relationship between noble gas compositions Figure 8 shows a ternary plot of a helium mass balance broken and geologic phenomena, such as young faults, tectonic occur- down into percentage of the three model parameters. The model rences like the Aspen anomaly, or volcanic activity such as the of the helium system parameters was generated from results by Valles caldera. defining the measured helium isotopic compositions as a percent between the atmospheric, crustal, and magmatic end-members. Conclusions The highest atmospheric component is observed in the Big Ar- senic Spring near Questa, New Mexico and a crustal component At the Poncha Pass field area, the Hot Springs Road geophysi- ranging from 57 to 97 % is present in all other samples. The cal surveys identified two faults that coincide with evidence of sample collected from Poncha Springs during this research, and upwelling water. Water chemistry collected at each site is consis- data from Karlstrom et al. (2011) contain a high percentage (21-26 tent with previously collected data, and the noble gas data indicate %, respectively) of the magmatic end-member. Finally, Figure a direct and unique relationship between geological phenomena and noble gas composition. Elevated mantle noble gas signatures are found near areas of extensive structural deformation, such as the Villa Grove transfer fault zone, and proximal to tectonic or volcanic occurrences, such as the Aspen anomaly and Valles caldera, respectively. Thus, noble gases may potentially serve as a useful tool for geothermal exploration because they indicate the presence of deeply penetrating geologic structures and, in some cases, sources of mantle-derived heat. Further work to continue noble gas isotope and bulk gas sampling in the San Luis basin is scheduled. Future bulk gas !"#$%&'()*+*,-( results will allow for more a realistic modeling approach to simulate geothermal reservoir mixing processes, and additional noble gas isotope data will enhance the current conceptual model of the relationship between noble gas isotope compositions and Figure 7. RC/RA vs Excess Helium. geological occurrences.

765 Easley, et al.

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